1. Field of the Invention
The present invention relates to an image display system, in particular, an on-screen input image display system that enables to detect multipoint coordinates in a touch sensor of a capacitance coupling type.
2. Description of the Related Art
An image display device including a touch sensor (also referred to as touch panel) having an on-screen input function for inputting information to a display screen by a touch operation with a user's finger or the like (contact and press operation, hereinafter, simply referred to as “touch”) is used for a mobile electronic device such as a PDA or a cellular phone, various home electric appliances, a stationary customer guiding terminal such as an automatic reception machine, and the like. As to the image display device having such a touch input function, there are some known methods including a method of detecting a change in resistance value or capacitance of a touched part, a method of detecting a change in quantity of light at the part shielded by the touch operation, and the like.
It is desired for the image display device including the touch sensor to have a function of multipoint touch detection so that the usability can be improved and that a new application (e.g., image scaling function and the like) can be realized.
FIG. 14 is a structural diagram for explaining an example of a conventional capacitive touch sensor. This touch sensor is called a two dimensional capacitive sensor. For convenience of description, the touch sensor includes a detection cell in which electrodes of five rows (in the X direction) and three columns (in the Y direction) are arranged in two-dimensional matrix via an insulator layer. In FIG. 14, the detection cells are distinguished from each other by their contours. Here, the detection cell means an electrode region for detecting capacitance of a finger or the like. As to the detection cells in the column (e.g., detection cell 84), a column detection electrode (Y coordinate electrode) passes through the detection cells continuously as a back column (linkage electrode), and a row detection electrode (X coordinate electrode, e.g., detection cell 86) is made up of two conductive regions on both sides of the column detection electrode, which are connected by electric wire connection 41.
As to the detection cells in the end column of the detection region (i.e., columns X1 and X3, e.g., detection cell 86), the row detection electrode passes through the detection cells continuously, and the column detection electrode is made up of two conductive regions on both sides of the row detection electrode. In this structure, the column detection electrode and the row detection electrode in each detection cell are connected electrically to each other. The row detection electrodes on both ends of the detection cell are connected to each other via electric wire connections 38, 40, and 41 formed on the outside of the detection region, and hence the electric wire connection does not need to cross the detection region. In other words, it is possible to provide a capacitive position sensor including the detection region having the electrode only on one side of a substrate.
In addition, as for the multipoint detection, there is an input device disclosed in Japanese Patent Application Laid-open No. 2006-179035. The input device disclosed in Japanese Patent Application Laid-open No. 2006-179035 is made up of a first switch group for applying a signal from an oscillator to one direction electrodes (X electrodes) for detection arranged in two-dimensional matrix, a second switch group for retrieving a signal of the other direction electrodes (Y electrode), and a signal detection circuit (AM modulation circuit). In this structure, the signal from the oscillator is supplied to one X electrode selected by the first switch group, and in this state, the second switch group selects the Y electrodes one by one sequentially, whereby the AM modulation circuit detects whether or not an increase in capacitance has occurred in the X electrode. In addition, the X electrode to which the signal from the oscillator should be applied is selected sequentially. This operation is repeated, thereby detecting a change in capacitance of the touched part.